Apparatus for polishing thin, flat semiconductor wafers are well-known in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semiconductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head, a wafer unload station, or a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is “planarized” or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A schematic of a typical CMP apparatus is shown in FIGS. 1A and 1B. The apparatus 10 for chemical mechanical polishing consists of a rotating wafer holder 14 that holds the wafer 10, the appropriate slurry 24, and a polishing pad 12 which is normally mounted to a rotating table 26 by adhesive means. The polishing pad 12 is applied to the wafer surface 22 at a specific pressure. The chemical mechanical polishing method can be used to provide a planar surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films.
CMP polishing results from a combination of chemical and mechanical effects. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An altered layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide may be formed and removed separately.
A polishing pad is typically constructed in two layers overlying a platen with the resilient layer as the outer layer of the pad. The layers are typically made of polyurethane and may include a filler for controlling the dimensional stability of the layers. The polishing pad is usually several times the diameter of a wafer and the wafer is kept off-center on the pad to prevent polishing a non-planar surface onto the wafer. The wafer is also rotated to prevent polishing a taper into the wafer. Although the axis of rotation of the wafer and the axis of rotation of the pad are not collinear, the axes must be parallel. Polishing pads of the type described above used in the CMP process are shown in U.S. Pat. No. 4,141,180 to Gill, Jr., et al.; U.S. Pat. No. 5,205,082 to Shendon, et al; and U.S. Pat. No. 5,643,061 to Jackson, et al. It is known in the art that uniformity in wafer polishing is a function of pressure, velocity and the concentration of chemicals. Edge exclusion is caused, in part, by non-uniform pressure on a wafer. The problem is reduced somewhat through the use of a retaining ring which engages the polishing pad, as shown in the Shendon et al patent.
The polishing pad 12 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surfaces. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
Referring now to FIG. 1C, wherein an improved CMP head, sometimes referred to as a Titan head, which differs from a conventional CMP head in two major respects, is shown. First, the Titan head employs a compliant wafer carrier and second, it utilizes a mechanical linkage (not shown) to constrain tilting of the head, thereby maintaining planarity relative to a polishing pad 12, which in turn allows the head to achieve more uniform flatness of the wafer during polishing. The wafer 10 has one entire face thereof engaged by a flexible membrane 16, which biases the opposite face of the wafer 10 into face-to-face engagement with the polishing pad 12. The polishing head and/or pad 12 are moved relative to each other, in a motion to effect polishing of the wafer 10. The polishing head includes an outer retaining ring 14 surrounding the membrane 16, which also engages the polishing pad 12 and functions to hold the head in a steady, desired position during the polishing process. As shown in FIG. 1C, both the retaining ring 14 and the membrane 16 are urged downwardly toward the polishing pad 12 by a linear force indicated by the numeral 18 which is effected through a pneumatic system.
An exploded, perspective view of a Titan CMP head 20 is shown in FIG. 1D and includes an upper assembly or housing 22 and a lower assembly 23 having a flexure clamp 24, a diaphragm flexure 26, a membrane clamp 28, a membrane support 30, a flexible membrane 32, and a retainer ring 34. The membrane 32 is mounted on the bottom surface of the membrane support 30. In assembly of the CMP head 20, the diaphragm flexure 26 is clamped on the membrane support 30, between the flexure clamp 24 and the membrane clamp 28, as follows. First, the membrane clamp 28 is placed on the membrane support 30, with multiple screw openings 29 that extend through the membrane clamp 28 registering with respective screw openings (not shown) provided in the upper surface of the membrane support 30. Next, the diaphragm flexure 26 is placed on the membrane clamp 28, with multiple screw openings 27 that extend through the diaphragm flexure 26 registering with the respective screw openings 29 in the membrane clamp 28. Next, the flexure clamp 24 is placed on the diaphragm flexure 26, with multiple screw openings (not shown) that extend through the flexure clamp 24 registering with the respective screw openings 27 in the diaphragm flexure 26. Finally, screws 36 are extended through the respective screw openings (not shown) in the flexure clamp 24, the screw openings 27 in the diaphragm flexure 26, the screw openings 29 in the membrane clamp 28 and the screw openings (not shown) in the membrane support 30. The lower assembly 23 and the retaining ring 34 are then mounted inside the upper assembly 22 to complete assembly of the Titan CMP head 23.
A drawback that is frequently encountered in assembling the Titan CMP head 20 is difficulty in facilitating proper alignment of the screw openings 27 of the diaphragm flexure 26 with the screw openings 29 of the membrane clamp 28 prior to extension of the screws 36 through the screw openings 27, 29. This is so due to the elastic and sticky characteristics of the diaphragm flexure 26. In the event that the screw openings are not properly aligned through the membrane clamp 28, the diaphragm flexure 26 and the flexure clamp 24, the screws 36 tend to damage the diaphragm flexure 26, thereby causing leakage of compression air between the membrane 32 and the upper assembly 22. Consequently, the downward pressure applied against the polishing pad (not shown) by the membrane 32 is unstable, thus compromising the CMP polishing removal rate and polishing profile on the wafer surface. Moreover, since the membrane vacuum pressure is important for holding the wafer against the membrane during wafer loading and unloading, loss of the vacuum pressure resulting from air leakage in the CMP head can result in premature falling of the wafer from the CMP head. Accordingly, a new and improved diaphragm flexure is needed to facilitate the precise alignment of screw openings in the diaphragm flexure with respective screw openings in the other elements of the lower assembly in order to provide proper sealing engagement between the membrane and the upper assembly of the CMP head.
An object of the present invention is to provide a new and improved diaphragm flexure for a CMP head.
Another object of the present invention is to provide a new and improved diaphragm flexure which is particularly suitable for Titan (TM) CMP heads.
Still another object of the present invention is to provide a new and improved diaphragm flexure which facilitates ease and accuracy in assembling a CMP head.
Yet another object of the present invention is to provide a new and improved diaphragm flexure which provides an airtight seal between a membrane and an upper assembly or housing of a CMP head to achieve optimal polishing removal rate and polishing profile on a wafer.
A still further object of the present invention is to provide a new and improved diaphragm flexure which includes an annular flexure body having at least one upper bulge or protrusion that mates with a companion groove provided in an element or elements of a CMP head and at least one upper bulge or protrusion which mates with a companion groove provided in another element or elements of the CMP head to facilitate proper alignment and sealing of the elements in the CMP head and achieve optimal vacuum pressure applied to a membrane in the CMP head for obtaining a uniform polishing rate and profile on a wafer.
Yet another object of the present invention is to provide a method of preventing or minimizing air leakage between a membrane and an upper assembly or housing of a CMP head.